|Normal||True Vault Hunter|
Dr. Zed will tell you about these weird wounds that his patients have been suffering from. Particularly, it appears as though they are being hit by bullets but there is no trace of the bullet itself. He will then send you to find Dr. Mercy to find out what's going on.
Travel to Three Horns - Valley and go to the waypoint on your map. This will lead you to the Shock Fossil Cavern and the lair of Dr. Mercy. There are two ways into this cave but it is otherwise symetrical. When you enter the cave, walk straight ahead to come to a large round room with an equally large electrical tower in the middle of it. At this point, be on the look out for bandits, particularly Marauders, Psychos, and Nomads. When you reach this room, there will be a path directly ahead of you and one off to the side (depending on which way you came in).
Kill any bandits in the immediate vacinity and then start making your way to this side path. Take it very slow at this point because there's a lot of bandits packed into this small space. Move from cover to cover killing the enemies in front of you (a fire weapon would be helpful if you have access to one). Thankfully, since you cleared the round room behind you, they can only come straight at you so you can just focus on them rather then worry about being flanked.
After you've cleared out the minor bandits, make your way up to the back of the area. At the very top of the stairs at the back, you'll find Dr. Mercy. He's a Nomad style enemy with a very large shield, an energy shield, and a gun that shoots energy at you. When you first see him, start backing up to get behind cover but also try and shoot at his feet as he comes down the stairs to get him started. Even if his minions were easy to kill, his weapon is going to be very strong so you're going to want that cover.
Keep backing up from cover to cover as he gets closer. Like most Nomad type enemies, you're going to want to try and get behind him or knock him off balance with a powerful shot before following up with a few shots to the flesh. When he's dead, loot his body for an E-Tech weapon.